In order to avoid cavitation whilst retaining a large adjustment range and a rapid response time even with large pressure differentials over the main piston, which divides a shock absorber into two damping medium-filled chambers, a compression chamber and a rebound chamber, use is currently made of a principle which is referred to by the applicant as so-called positive pressure build-up or TTX. A pressurized damper with positive pressure build-up always has a pressure greater than zero on both sides of the main piston arranged in the damping cylinder, regardless of what conditions prevail. Pressurization on both sides of the main piston is achieved in that a gas pressurized member, arranged in a pressurization reservoir, pressurizes a damping medium-filled chamber common to both damping chambers to a predetermined pressure. Pressurization of the damping medium in the body of the damper can then be said to occur parallel with the movement, since the pressurized common chamber has a parallel connection to both of the damping chambers. The two damping chambers can be connected to the common chamber by means of a first flow duct between the compression chamber and the common chamber and a second flow duct between the rebound chamber and the common chamber. Two separate adjusting elements are provided in order to adjust the flow in the flow ducts between the respective damping chamber and the pressurized common chamber. Said adjusting elements comprise one or more damping force-generating, one-way valves(s) and a check valve, which opens as soon as the pressure in the common chamber exceeds the pressure in the chamber having the lowest pressure at that particular instant.
See the applicant's own patent application EP 1659310 A2, for example, in which a solid piston is used and in which the pressurization reservoir is arranged in a defined space inside the damping cylinder. Arranging the pressurization reservoir in line with the body of the damper results in a long overall length of the damper, and requires a complicated manufacturing process in order to create the flow ducts between the pressurization reservoir and each damping chamber.
Öhlins TTX system has also been used in a type of damper which has an external reservoir, a so-called “piggyback”. The external reservoir is then arranged outside but connected to the damping cylinder, and the internal damping medium-filled volume of the damping cylinder is connected to the interior of the reservoir via ducts defined by separate adjusting elements.
Some of these types of damper are designed to be capable of absorbing rapid movements and heavy impacts, caused by the nature of the running surface with its stones, holes and large irregularities. The limitation of the TTX principle in this type of damper lies in the area of the flow ducts and their capacity to transmit a sufficiently large flow of damping medium. In the prior art this limitation is solved by allowing a flow over the main piston in the event of large pressure differentials. The flow of damping medium is therefore parallel in the event of small pressure differentials and linear in the event of large pressure differentials, which means that in order to modify the damping characteristic of the shock absorber, adjustments need to be made to at least two adjustment points in order to adjust both the high-speed and the low-speed damping. In addition, it has proved difficult to fully control the pressure balance in a damper having two adjustment points. Further disadvantages are that a complete modification of the damping characteristic requires dismantling of the damper, whilst the pressure in the pressurizing reservoir has to be adjusted and balanced with the pressure in the damping chambers.
Another known solution is disclosed in the patent EP 1505315, which describes a damper having an external reservoir in which the damping chambers are separated from one another by a main piston, which at certain pressure differentials over the main piston allows a flow through the main piston. The damper also has two separate adjusting valves, designed to adjust the flow in flow ducts extending between the respective damping chamber and a pressurized damping medium-filled chamber in the external reservoir. Under a compression stroke, the compression chamber is pressurized via a first adjusting valve to a pressure that prevails in an external pressurization vessel, so as to prevent a negative pressure in the chamber. Under a rebound stroke the second adjusting valve is used to adjust the damping characteristic. In this damper also, a flow is allowed over the main piston and the ducts that extend between the valves and the respective damping chamber are only adjusted to the flow that prevails at small to medium damping speeds.
With a TTX damper having a solid main piston, like that in EP 1659310 A2, all the damping medium subjected to the area of the main piston acting on the damping medium is forced through the respective flow duct. Adjustment of the damping characteristic is then simplified in that only the externally located adjusting elements/the valves are used for this adjustment. This means, however, that the ratio between the area of the main piston and the area of the flow ducts becomes critical for the flow of damping medium through the valves. In addition, it is desirable, in order to ensure that a positive pressure prevails in the damping chambers and to obtain the same valve characteristic in both valves, that one and the same pressure should act on both of the valves without introducing unnecessary restrictions into the shock absorber.
In order to produce a compact TTX damper, valves having a smaller diameter/area than the diameter/area of the main piston are used. Since the area of the valve is smaller than the area of the solid main piston, this means that a restriction nevertheless has to be introduced into the system somewhere or other.
Creating a restriction of the damping medium flow in a vehicle damper having a positive pressure build-up and a solid main piston, without a turbulent flow occurring, has proved to be a problem.
There can also be a problem, when using a valve diameter smaller than the diameter of the main piston, in that the variation in the rate of flow via the damping ducts needs to be so great that disturbances and fluctuations can occur during movement of the oil. Valve resonance can also occur, which can lead to jarring and an impaired damping function.
It can moreover be difficult to solve these problems without the damper assuming excessive dimensions, especially in a longitudinal direction, that is to say from fixing point to fixing point.